In recent years, problems related to aging of structures such as bridges constructed during the period of high economic growth have become noticeable. Because loss is immeasurable when an accident occurs in a structure, technologies for monitoring a state of a structure have been proposed. For example, a technology for detecting damage to a structure by an acoustic emission (AE) method in which an elastic wave generated due to occurrence of an internal crack or development of an internal crack is detected by a high-sensitivity sensor has been proposed. AE is an elastic wave generated due to development of fatigue crack of a material. In the AE method, an elastic wave is detected as an AE signal (voltage signal) by an AE sensor using a piezoelectric element. The AE signal is detected as an indication before breakage of the material occurs. Therefore, the frequency of occurrence of AE signals and the signal intensity are used as an index indicating the soundness of the material. For this reason, studies are being carried out on technologies for detecting signs of deterioration of structures by the AE method.
When a load doe to traffic or the like is applied on a concrete floor slab of a bridge, AE occurs due to crack propagation, friction, or the like in the floor slab. By installing an AE sensor on a surface of the floor slab, the AE generated from the floor slab can be detected. Moreover, by installing a plurality of AE sensors, a position of an AE source can be located from a difference in arrival time of AE signals between the AE sensors. A degree of damage to a target floor slab is estimated from the result of locating the position of the AE source. However, when the correspondence between the location result and the degree of damage is not sufficient, stable soundness evaluation cannot be performed in some cases. Such a problem is not limited to concrete floor slabs of a bridge but is a problem common to all structures in which elastic waves are generated as cracks occur or develop.